Autocatalytic effect, absence

The autocatalytic effect of lead on the rate of reactions (3) and (4) was demonstrated146 by comparing the pyrolysis of mixtures of tetramethyl lead and hexaethyldiplumbane and of hexaethyldiplumbane and diethyl lead in the presence and absence of lead formed by reaction (4). 

At temperatures above 30°C, the rate increased slightly with temperature in a cylindrical reaction vessel but decreased slightly in a spherical vessel. A sharp increase in rate was noted as the temperature was decreased from 30°C. In the absence of water vapor or nitrogen dioxide a distinct induction period was observed, during which products were formed very slowly. The apparent rate increased slowly, and reached a maximum one-fourth to one-third of the way to reaction completion. The results quoted below were obtained near the start of the reaction but after the apparent induction period. The autocatalytic effect of nitrogen dioxide had been observed earlier in aqueous solutions. The reaction was found to obey the following rate law reasonably well. 

Let us emphasize the following points in closing this description of a very generally useful reaction rate expression. First, the orders of reaction and have no necessary connection with the stoichiometric coefficients but must be determined experimentally. Second, they are, however, con-- jSj = and in the absence of autocatalytic effects by > 0- For an elementary step of a reaction, however, the order... 

In the simple example considered in the last paragraph, we saw that r decreased as increased. Let us show that in the absence of any autocatalytic effects, this is always the case. It is useful to recall (see Exercise 3.7,2) that... 

The authors attribute the observed selectivity to an autocatalytic effect arising from the rapid production of secondary nuclei from a single primary nucleus in the stirred system. In essence, they propose that the first crystal to form will be either an L-crystal or o-crystal, each with 50% probability. When the solution is stirred, this ur-crystal (i.e., original crystal) serves as the template or the source for the formation of other crystals from solute brought to it by the stirring-induced transport in the supersaturated solution. In the absence of stirring, crystals form independently—there is no communication between crystals already formed and the rest of the solution, except for that provided by the relatively slow process of diffusion. 

By sonically inducing the polymerisation of methyl methacrylate (MMA), Price [65] has extended the work of Kruus and studied the effect of the absence and presence of the initiator azobisisobutyronitrUe (AIBN). Similar conversions to Kruus (2-3% per hour) were obtained in the absence of initiator at 25 °C. However considerable improvements in the polymerisation rate were observed when 0.1% of initiator were used (Fig. 5.40), the reaction appearing to become autocatalytic. This no doubt is due to the faster production of polymer in the initiated system (faster initiation due to enhanced initiator breakdown) which is then available for degradation to produce more free radical entities. 

In the early nineteen-sixties Halpem, James and co-workers studied the hydrogenation of water-soluble substrates in aqueous solutions catalyzed by rathenium salts [6]. RuCh in 3 M HCl catalyzed the hydrogenation of Fe(III) to Fe(II) at 80 °C and 0.6 bar H2. Similarly, Ru(IV) was autocatalytically reduced to Ru(III) which, however, did not react further. An extensive study of the effect of HCl concentration on the rate of such hydrogenations revealed, that the hydrolysis product, [RuCln(OH)(H20)5. ] " was a catalyst of lower activity. It was also established, that the mechanism involved a heterolytic splitting of H2. In accordance with this suggestion, in the absence of reducible substrates, such as Fe(in) there was an extensive isotope exchange between the solvent H2O and D2 in the gas phase. 

A distinctive feature of calpain-3 is the propensity to autolysis the half life of the protein in vitro is less than 30 minutes. Autoproteolysis is prevented by replacing the cys residue of the catalytic site with a ser, but is not affected by cysteine protease inhibitors such as calpastatin, E64 and Leupeptin (Sorimachi et al., 1993). The autocatalytic activity and the absence of effective inhibitors have made the... 

Since many biochemical reactions and their stoichiometry are not well understood, we often find a more empirical approach to the quantitative assessment of the kinetics. Mass concentration units (e.g., g/L) are often used along with yield coefficients to calculate the distribution of products formed and the amount of substrate consumed. In the absence of any inhibition effects and in the presence of an infinite supply of substrate, the rate of cell growth rx is autocatalytic, that is, it depends only on the concentration of cells (Cx), and the more cells we have, the higher the growth rate. The cell biomass is typically represented by X ... 

There is an early report in the literature claiming absence of the autocatalytic reaction enhancement in TS if the reaction is induced by UV-excitation of the monomer crystal. The implication would be that thermal and UV-polymerization involve different mechanisms. Later on, however. Chance and Patel found this to be an artifact caused by the neglect of spatially inhomogeneous absorption by polymer molecules which effectively competes with monomer excitation at increasing conversion and prematurely terminates the reaction. Although it is difficult to correct X(t)-curves obtained under UV-excitation for polymer absorption quantitatively, particularly if irradiation is done with unpolarized non-monochromatic light, it turns out that there is a qualitative agreement between X(t)-curves obtained under y-and UV-irradiation. Application of this correction, however, does not solve the puzzle why in case of y- or UV-polymerization of TS, the reaction rate increases less dramatically with conversion, than observed upon thermal conversion. 

The linear stability analysis of eqns (2.30) shows that the domain of instability of the unique steady state is larger than in the case of a linear sink of product. The main effect of the Michaelian sink of product is, however, to allow the occurrence of sustained oscillations in the absence of enzyme cooperativity - but not of autocatalytic regulation (Goldbeter Dupont, 1990). 

The absence of evidence for homogeneity has prevented a clarification of the dual specificity of the phenol oxidases. Most preparations oxidize both monophenols and o-diphenols. With the monophenols, however, the maximum oxidation rate with purified enzymes is always preceded by an induction period which can be eliminated by the addition of traces of diphenol.This autocatalytic behavior with monophenols and the priming effect of diphenols may be due to a catalytic role of diphenols formed in the oxidation of the monophenolic substrate.Thus the oxidation of monophenols appears to follow the pattern ... 

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